Before the discovery of auxin, a theoretical compound termed rhizocaline was postulated to cause root formation in cuttings.
Much evidence supports auxin as the key hormone that induces rooting. However, in many cases application of auxin is insufficient to promote rooting.
For that reason, it is felt that other substances working with auxin (rhizocalines) may be required to induce rooting.
It could be shown experimentally that cuttings with leaves rooted better than those with the leaves removed.
Van Overbeek in 1946 reported that grafts between easy and difficult-to-root cultivars of rose of Sharon (Hibiscus rosa-sinensis) resulted in a graft transmission of a substance from the easy-to-root cultivar that improved rooting in the difficult-to-root cultivar.
Reciprocal grafts between juvenile and mature English ivy shows that a substance from the juvenile leaf improves rooting in the mature petiole.
The search continues for rhizocaline.
For example, Max Kawase in the 1970's found that an extract from willow stems could increase rooting in stem cuttings from a variety of species.
The chemical nature of willow extracts has not yet been established.